Igf1r signalling acts on the anagen-to-catagen transition in the hair cycle.

Insulin-like growth factor 1 (Igf1) is important for skin development and homoeostasis. However, overexpression and inactivation studies have produced variable findings regarding its role in hair follicle (HF) biology. Here, we studied a conditional and inducible knockout of the Igf1 receptor (Igf1r) in keratin 15-expressing bulge cells. Deletion of Igf1r after the development of the skin appendages in K15-Igf1rKO mice showed no abnormalities in epidermal homoeostasis. Numbers of bulge cells were lower in K15-Igf1rKO mice than in controls, without consequences on wound healing, at least in young mice. K15-Igf1rKO HFs entered anagen phase earlier than controls and showed a delay in the anagen/catagen switch. The expression of Bmp-4 mRNA was inhibited in HFs from K15-Igf1rKO . MED1 transcription was impaired in the epidermis of K15-Igf1rKO mice. These findings suggest that Igf1r controls the hair cycle, partly through Bmp-4 activation.